Tests identify fever that killed 15 million

Salmonella discovered in DNA of victims of the 16th-century epidemic

Scientists have narrowed down the cause of death of as many as 15 million people during an epidemic in Mexico nearly 500 years ago, contributing to the destruction of the Mexica, or Aztec, empire.

DNA tests have revealed that the second of two epidemics that the locals called a cocolitzli, the Náhuatl word for pestilence, wasn’t measles, mumps or influenza as suspected but salmonella enterica, which causes enteric or typhoid fever.

The epidemic struck in 1545; within five years up to 15 million people — an estimated 80% of the population — had died.

“The 1545-50 cocoliztli was one of many epidemics to affect Mexico after the arrival of Europeans, but was specifically the second of three epidemics that were most devastating and led to the largest number of human losses,” said Åshild Vågene of the University of Tuebingen in Germany, co-author of a study published yesterday in Nature Ecology & Evolution.

“The cause of this epidemic has been debated for over a century by historians and now we are able to provide direct evidence through the use of ancient DNA to contribute to a longstanding historical question.”

It is considered one of the deadliest epidemics in human history, approaching the severity of the bubonic plague that killed 25 million people in Europe in the 14th century.

The first cocoliztli, recorded around 1525, is believed to have been smallpox and killed as many as 8 million people.

Another outbreak of smallpox occurred between 1576 and 1578, and killed half of the remaining regional population.

The diseases were spread by Europeans venturing into the new world, carrying germs against which the local population lacked immunity.

Symptoms of the second epidemic were high fever, headaches and bleeding from the eyes, mouth and nose, and death followed within three or four days.

The new study, conducted by researchers from the Max Planck Institute for the Science of Human History, Harvard University and the Mexican National Institute of Anthropology and History, identified the cause after the teeth of 29 victims — all buried in a cocolitlzli cemetery — were analyzed for DNA. Typhoid fever was the strongest candidate.

The researchers were even able to trace the presence of the same bacterial strain to today’s population, among whom it barely even causes infections now.

“We tested for all bacterial pathogens and DNA viruses for which genomic data is available,” and salmonella enterica, the bacteria that causes typhoid fever, was the only germ detected, said Alexander Herbig, co-author of the study.

The possibility remains that some pathogens were either undetectable or completely unknown, said the specialists.

“We cannot say with certainty that salmonella enterica was the cause of the [second] cocoliztli epidemic,” said team member Kirsten Bos. “We do believe that it should be considered a strong candidate.”

Many salmonella strains spread through infected food or water and may have traveled to Mexico with domesticated animals brought by the Spanish, the research team said.